1. Field of the Invention
The field of the present invention relates to wireless communication. Specifically, the field of the invention relates to the optical transmission of analog signals between antenna and a baseband processor via fiber.
2. Background of the Invention
Second generation (2G) cellular radio networks consist of three deployment hierarchy layers: the base transceiver station (BTS), the base station controller (BSC), and the mobile switching center (MSC). The part comprising BSC, BTS, and antenna is called the base station subsystem (BSS). The evolution of the BSS has been a major focus of the radio access network (RAN) standardization by Special Mobile Group 5 (SMG5) and later 3rd Generation Partnership Project (3GPP).
Third generation (3G) cellular radio networks inherited this architecture, albeit simplified, in which the BSC is removed (i.e., the function is split into the MSC and the BTS), and the BTS is directly connected to the gateway of the core network. In practice, the remote radio unit (RRU) and the base band unit (BBU) are separated (i.e., functionally separated into two products). As such, the intensive software processing of the base band data is separated from the high frequency (HF) signal reception. The most used interface between the RRU and the BBU is Common Public Radio Interface or Ir. For 2G systems, RRUs are mostly collocated with BBUs. But for 3G systems, RRUs are separately located (i.e. RRUs are installed on top of transmit towers, while BBUs are installed in equipment rooms at the foot of the towers). In this way, one BBU can support multiple RRUs, and so digital signal processing facilities can be concentrated. FIG. 1 shows an overview of 3G wireless network architecture and FIG. 2 shows 3G RAN architecture. Both figures are shown according to 3GPP standards.
Fourth generation (4G) cellular radio networks continue this trend by further flattening the architecture. For example, an evolved nodeB (eNB) is directly connected to the mobility management entity (MME) (core network), and multiple antennas in different locations can be connected to eNB. FIG. 3 illustrates 4G RAN architecture.
The trend is continued by the Cloud-RAN (C-RAN) concept, where optical fibers are large-scale deployed to further concentrate the BBU resources. As such, more and more RRUs can be supported by a single BBU, allowing the internet protocol (IP) technology of cloud computing to enhance processing power and flexibility. This results in reduced power consumption and dynamic resource allocation in the cellular network. FIG. 4 illustrates C-RAN architecture.
As shown in FIG. 5, C-RAN is characterized by the aggregation of multiple cell sites via optical fibers and centralized base band processing. C-RAN architecture saves structures, power supplies, and maintenance at the cell sites, while at the same time enabling better inter-cell/sector cooperation and computing facilities. The technical solution is provided by utilizing a new layer-0 (i.e., the layer below the physical layer (according to the International Standard Organization (ISO) communication protocol model)). Because layer-0 is not part of the 3GPP's scope, C-RAN is a non-standard solution.
As further shown in FIG. 5, RRUs are directly connected to the antenna system and, as such, the inputs of the RRUs are raw HF signals. The output of the BBU is base band data needed by the eNodeB/BSC, and hence belongs to Media Access Control (MAC) layer or above. The data between the RRUs and the BBU can take different formats depending on the functional definition and separation between the BBU and the RRUs. FIG. 6 shows the location of C-RAN interface in the protocol stack architecture of 3GPP and a RAN protocol stack.
The interface comprises a transformation to or from HF and then to or from base band (I-Q) digits. The chain of processing essentially consists of four operations: (i) RF-IMF/IMF-RF (where IMF—intermediate frequency—is a digital intermediate frequency signal); (ii) modulation/demodulation; (iii) sampling; and (iv) analog-to-digital conversion/digital-to-analog conversion (ADC/DAC). The four operations can be allocated in either RRU or BBU—the criterion is feasibility and the efficient transportation of the information. A current interface approach is through the so-called CPRI interface. The CPRI standard defines the interface of base stations between the Radio Equipment Controllers (REC) in the standard, to local or remote radio units, known as Radio Equipment (RE).
Functional separation according to the CPRI interface is (1) RRU: RF-IMF/IMF-RF, sampling, ADC/DAC; and (2) BBU: Process I-Q data stream. By this scheme, in the up-link, the modulated RF data coming from the antenna feeder are first down converted to base band where they are digitized with fixed sample rate. The digits are then packetized according to the CPRI protocol. Fiber is used to transport the CPRI packets to the central processing location. FIG. 7 shows CPRI functionality and protocol stack and FIG. 8 shows a digital RRU-BBU link.
A critical issue is the timing of CPRI packets from different RRU locations and the synchronization between the BBU and the connected RRUs. This is particularly difficult for the approach involving CPRI, or any other packet-based transmission protocol, because the data consists of digitized baseband analog signal samples (IQ data) that require very high transmission capacity and time accuracy.
The prior art uses a small form-factor pluggable (SFP) at the optical interface to convert the electrical sample data into optical data. As such, a single fiber is required for each RRU. To improve efficiency, wavelength division multiplexing (WDM) has been deployed to allow sharing of the optical capacity of a single fiber between different RRUs. Despite all however, the CPRI-based aggregation approach is flawed.
First, the HF data has to be digitized at the RRU, creating a huge amount of unprocessed samples subject to transmission over the fiber. As a result, fiber capacity is inefficiently utilized (or quickly exhausted) because raw data is transmitted, as opposed to data from the upper layer; for example, the A-Interface for the backhaul. Second, because of the high data rate, the synchronization is much more stringent than upper layer data, which has more delay budget to absorb any disparity in the synchronization. Third, the transmission chain contains redundant components because analog signal are converted to digits, then the electrical digits are converted to optical analog signals, and then the optical analog signals are again converted into electrical digits. The high capacity of digital data in the middle of the transmission chain adversely affects the transmission system.
In addition, operators are interested in utilizing the Gigabit—capable Passive Optical Network (GPON) access network to carry the CPRI data, but difficulty is encountered because the capacity of GPON can hardly keep up with the demand, leaving no room for sharing with other services, such as fiber to the x (FTTx). Finally, sharing a digital channel in the same infrastructure also causes security concerns for operators. This and other circumstances present problems and obstacles that are overcome by the methods and systems described below.